This application claims the priority benefit of Taiwan application Ser. No. 88103039, filed Mar. 1, 1999, the full disclosure of which is incorporated herein by reference.
1. Field of Invention
The present invention relates to a method for cleaning the interior of an etching chamber. More particularly, the present invention relates to a method for cleaning the interior walls of an etching chamber with plasma.
2. Description of Related Art
In the manufacture of semiconductor products, most gate conductive layers of a MOS transistor are composed of a doped polysilicon layer and a metallic silicide layer. The metallic silicide layer/is probably a tungsten silicide (WSix) layer. In general, the transistor gate structure is formed by first depositing a polysilicon layer and then a metal silicide layer over a substrate. Next, a photoresist layer is deposited and then patterned using a photolithographic method. Finally, the metal silicide layer and the polysilicon layer are sequentially etched using an anisotropic plasma etching method to form a gate structure.
An anisotropic plasma etching operation is a process in which highly energetic particles within the plasma are deployed to bombard tungsten silicide and polysilicon of the conductive layer. Therefore, some reaction products are deposited on the interior walls of the reaction chamber and gradually accumulate. These reaction products, composing mainly micro-particles or high molecular weight compounds, may drop back onto the surface of the silicon wafer when the wafer is undergoing an etching operation. Often, this may lead to the deviation of critical dimensions for some of the devices on the wafer.
In consequence, after the reaction chamber has been used for a number of times or the etching station has been left idle for more than an hour, a dry cleaning process needs to be carried out. The dry cleaning process tries to remove any deposited reaction products attached to the interior walls of the chamber, thereby maintaining a rather constant reaction environment inside the chamber and reducing product yield problems.
A dry cleaning process actually comprises a dry cleaning operation and a warm-up operation. Conventionally, the dry cleaning operation requires the execution of an etching operation inside the reaction chamber for about 500 RFmin. In other words, the process includes powering up at normal working radio frequency (RF) range for about 500 minutes with three silicon wafers inside the reaction chamber. Next, a mixture of gases including chlorine (Cl2) and carbon hexafluoride (CF6) are passed into the chamber to form a plasma that can be used to bombard the interior walls of the reaction chamber. The bombarding operation lasts for another 5 RFmin. Hence, most of the reaction products deposited on the interior walls of the reaction chamber are removed.
Since most reaction products deposited on the interior walls of the reaction chamber are removed during the dry cleaning phase, the environment inside the reaction chamber with respect to wafer etching may change suddenly. Great changes in the etching environment may lead to instability when etching a conductive layer. In order to obtain a more stable wafer quality, a warm-up operation is necessary to stabilize the reaction environment inside the chamber.
In a conventional warm-up operation, a dummy wafer is placed inside the chamber and then a reaction condition similar to a normal etching operation is applied. Since etching a conductive layer actually includes etching a photoresist layer, a tungsten silicide layer and a polysilicon layer, the warm-up operation must includes several steps. Firstly, a gaseous mixture of oxygen, hydrogen and hydrogen bromide (HBr) is passed into the chamber to form a plasma, and then the dummy wafer is bombarded using the plasma for about 90 seconds. Next, a gaseous mixture of chlorine and hydrogen bromide is passed into the chamber to form another plasma, and then the dummy wafer is again bombarded by the plasma for about 140 seconds. Finally, a gaseous mixture containing hydrogen bromide, helium and helium dioxide (HeO2) is passed into the chamber to form yet another plasma, and then the dummy wafer is bombarded with the plasma for about 100 seconds. The warm-up operation requires altogether some 3 to 10 dummy wafers. After the warm-up operation, a more stable etching environment is obtained.
However, in the aforementioned method of cleaning the reaction chamber, fluoride-containing (F) gaseous mixture is used as a plasma source. Therefore, a considerable amount of fluorine radicals is attached to the interior walls of the reaction chamber. When metal silicide or polysilicon layers are being etched, these fluorine radicals can react with the chlorine and hydrogen bromide in the plasma. Consequently, uniformity of wafer surface may worsen and the etching rate may be reduced, leading to larger deviations in the critical dimensions of some devices. In some cases, even an etching selectivity ratio between polysilicon and oxide may be affected. If the etching selectivity ratio changes, it is rather difficult to determine the etching end point when polysilicon is etched. Moreover, the plasma used for etching the polysilicon layer may etch into the gate oxide layer between the gate conductive layer and the substrate. Since the gate oxide layer is already a very thin layer of about 80 xc3x85, any further reduction of the gate oxide layer down to a thickness of about 40 xc3x85 or lower by etching leads to serious problems such as the pitting of the active area of a device.
The amount of fluoride radicals on the interior wall of the chamber is reduced gradually as the number of etching operations being conducted increases, and hence the etching environment recovers only after a while. Therefore, etch quality of the first few silicon wafers immediately after the dry cleaning process may not be too stable, but improves considerably due to the stabilization of the chamber environment later on. The phenomenon of the first batch of a few silicon wafers having a poorer quality than subsequent batches is called the first wafer effect or memory effect.
Furthermore, several pieces of silicon wafers and dummy wafers are wasted during the dry cleaning operation as well as the warm-up operation. Moreover, a wet cleaning operation needs to be carried out after every 8000 RFmin of etching operation despite the performance of dry cleaning processes from time to time. Hence, functional time of the reaction chamber is short and wafer productivity is low.
In light of the foregoing, there is a need to provide a better method of cleaning the reaction chamber.
Accordingly, the present invention provides a method for clearing away reaction products accumulated on the interior walls of an etching chamber after a number of etching operations so that stable etching operations can be resumed.
To achieve these and other advantages and in accordance with the invention, as embodied and broadly described herein, the invention provides a method for cleaning a silicon wafers etching chamber. The method includes performing a dry cleaning operation followed by a warm-up operation. In the dry cleaning operation, a gaseous mixture containing chlorine, oxygen and carbon hexafluoride is passed into the reaction chamber to form a first type of plasma. Next, the plasma is made to bombard against the interior walls of the reaction chamber. In the warm-up operation, a gaseous mixture containing the element hydrogen is passed into the reaction chamber to form a second type of plasma. Then, the plasma is again made to bombard against the interior walls of the reaction chamber so that fluoride free radicals inside the reaction chamber are removed. Ultimately, the etching environment within the etching chamber returns to normal.
The dry cleaning process according to this invention does not require the use of any silicon wafer. The dry cleaning process includes a dry cleaning operation and a warm-up operation. Hence, the method is capable not only of reducing first silicon effect and providing a stable etching environment inside the chamber, but also capable of minimizing silicon wafer wastes. In the dry cleaning process, only two types of plasmas are used, thereby having fewer steps and a shorter cleaning period. Furthermore, the reaction chamber can be used to etch silicon wafers for a longer period before cleaning is necessary. Hence, there is considerable increase in productivity.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.